1. Field of the Invention
The present invention is related to a method for the manufacture of thermoplastic materials containing polyphenylene ether (PPE) with a particularly low content of volatile components.
2. Discussion of the Background
It is known that thermoplastic materials can be manufactured by mixing in kneading machines in which the components are introduced in solid form, together or separately, in the desired mix ratios, and are mixed in the mixing zones during or after the melting process. In this process, the polymers are forcibly subjected to high mechanical and thermal stress. A method of this type yields unsatisfactory results with thermoplastic materials such as molding materials containing PPE, in which the admixed polymers are especially thermally and mechanically sensitive.
In order to avoid these disadvantages, it has been proposed, for example, in U.S. Pat. No. 3,383,435, to mix PPE and polystyrene at a lower temperature, particularly at temperatures between 232.degree. C. and 254.degree. C. Under these conditions, however, it is not possible to obtain a thorough mixture of both components. Therefore, even after a second extrusion, the objective is only a "more complete dispersion," but not a homogeneous mixture. If the temperature is raised to about 300.degree. C., so that the mixing effect is improved, then damage to the thermally sensitive polymers is unavoidable.
Another method is to decrease the thermal and physical stress by conducting the mixing process in solution or even in the presence of viscosity-reducing additives and then subsequently removing these agents from the remainder of the mixed material. Thus, it is known to produce solutions containing PPE and a vinyl-aromatic resin and to extrude them, if necessary after an upgrading process, through a degasification extruder (See CA-PS No. 1,063,761). This application contains no information whatever relating to the degree of degasification achieved or relating to the quality of the thermoplastic materials obtained.
Two additional patents contain embodiments of this method. According to DE-OS No. 31 31 440 a 15 to 60% PPE-solution is combined with another polymer and the combined solution is freed of solvents in a multiple-stage evaporation process. The method in DE-OS No. 33 15 803 provides for the upgrading of a 5 to 50% PPE solution, combining the same with the melted mass of another polymer at shearing velocities of from 5 to 400 sec. with subsequent or simultaneous upgrading through evaporation. In both methods the thermo-sensitive polymer is subjected to significant thermal stress in the upgrading step, since it is known that the final residue of a solvent is particularly difficult to remove from a polymer. The lower the residue level of volatile components in the polymer that is required, the higher the temperature must be and, therefore, also the mechanical stress on the polymer. If one chooses to operate in the reverse at relatively low temperatures during the upgrading process, then a high proportion of residual volatile components must be expected.
A way out of this dilemma is offered by the teachings of EP-PS No. 0 048 154. According to this reference, mixtures of polyphenylene ethers and impact resistant polystyrenes are melted in an extruder, mixed and degasified at temperatures of about 280.degree. C. The requirement for less than 5,000 ppm highly volatile components, however, can only be attained if the decomposition of the polystyrene components is suppressed by the addition of special inhibitors. But the presence of even 2,000-5,000 ppm of highly volatile components in such materials is still much too high, because during the manufacture of molded elements they cause the formation of clouded and streaked surfaces ("silver coating").